1. Field of the Invention
The present invention relates to a molding mixture for forming a long glass fiber reinforced polypropylene molding having high mechanical strength and to a molding formed of the molding mixture.
2. Description of the Related Art
Glass fiber reinforced polypropylene resins have been widely used for various industrial product components because they possess excellent mechanical strength, heat resistance and formability. Glass fiber reinforced polypropylene resins obtained by a drawing process in which continuous fibers are impregnated with a resin while being drawn (hereinafter referred to as "long" fiber reinforced polypropylene resins) exhibit superior impact strength, creep characteristics and vibration fatigue characteristics over glass fiber reinforced polypropylene resins obtained by kneading chopped strands and a resin with an extruder or molding machine (hereinafter referred to as "short" fiber reinforced polypropylene resins). Long fiber reinforced polypropylene resins produced by a drawing method also enables the binding of glass fibers at a very high density, because the glass fibers extend in a single direction in a bound state.
It is difficult to produce a short fiber reinforced polypropylene resin by kneading the resin with an extruder if the glass fiber content exceeds 50% by weight. On the other hand, long glass fiber reinforced polypropylene resin is easily produced by a drawing method even if the glass fiber content is 80% or higher by weight. Due to this advantage, long fiber reinforced polypropylene resins produced by drawing and having a large glass fiber content are frequently utilized as a master batch. When long fiber reinforced polypropylene resin possessing large glass fiber content is used as a master batch, the range from which the proportion of a diluting polypropylene mixed in the long fiber reinforced polypropylene resin is selected increases and, accordingly, melt process moldings of long fiber reinforced polypropylene having various fiber contents which are suitable for various kinds of use or purposes can be obtained. Thus, the use of long fiber reinforced polypropylene resins is economical because of their wide product application range.
However, long fiber reinforced polypropylene resins produced by drawing to be used as a master batch have drawbacks in that it is difficult to completely impregnate several thousands or several tens of thousands of glass fiber filaments with a polypropylene resin while drawing the filaments. Another drawback is that hydrophilic glass fibers have low wettability with non-polar polypropylene resins. Methods for reducing these drawbacks have been disclosed, as described below.
For example, Japanese Patent Laid-Open Publication 17631/1993 discloses a method of using a low-viscosity polypropylene of a melt flow rate of 30 g/10 min or higher measured by ASTM method D-1238 (load: 2.16 kg, temperature: 230.degree. C.) to facilitate molten resin impregnation, Japanese Patent Publication No. 25340/1991 discloses a method of impregnating glass fibers with a resin having a very small molecular weight, and Japanese Patent Laid-Open Publication No. 181528/1991 discloses a method of finishing the surface of glass fibers and using a modified polypropylene to improve the wettability.
Methods for obtaining moldings by a master batch process have also been disclosed, as described below.
Japanese Patent Laid-Open Publication No. 241406/1989 discloses a method of molding a dry blend of a long fiber reinforced thermoplastic resin and a thermoplastic resin having no long fibers, Japanese Patent Laid-Open Publication No. 114705/1991 discloses a method of using a polypropylene resin of a melt flow rate of 20 to 60 g/10 min or higher and pellets having a small size, i.e., a shape in which the length of at least one side is 1 mm or less and the specific surface area is 20 cm.sup.2 /g or larger, and Japanese Patent Laid-Open Publication No. 20339/1984 discloses a method of heating a combined filament yarn fabric of glass fiber and modified polypropylene fiber.
However, in prior art processes using a long glass fiber reinforced polypropylene resin as a master batch, a molding obtained by diluting such a resin and by molding the mixture tends to decrease in strength as glass fiber content increases. This tendency intensifies when the glass fiber content exceeds about 60% by weight.
The above-described prior art has been discussed with respect to impregnation techniques, particularly, methods for increasing fiber content, improving wettability, and obtaining moldings from master batches.
However, the above-described problems experienced when using a long glass fiber reinforced polypropylene resin as a master batch have not been addressed by the prior art and thus no method for solving the problem has been disclosed. If the problem of strength reduction in a molding obtained by using such a master batch is solved, moldings of long glass fiber reinforced polypropylene resins which satisfy the requirements for a wide array of uses and purposes could be produced at a low cost.